Generally, a conventional automatic transmission used in a vehicle includes a torque converter, a multistage gear shift mechanism connected to the torque converter, and a plurality of friction elements actuated by hydraulic pressure for selecting a gear shift stage of the gear shift mechanism.
The conventional automatic transmission is provided with a hydraulic control system which controls the automatic transmission.
In such a hydraulic control system, hydraulic pressure generated by a hydraulic pump is selectively supplied to each friction element by a plurality of control valves such that automatic shifting is realized in accordance with a driving state of the vehicle and engine throttle opening.
The above described hydraulic control system generally comprises a line pressure controller for regulating hydraulic pressure generated by the hydraulic pump to line pressure, a damper clutch controller for actuating a damper clutch of the torque converter, a reducing pressure controller for reducing line pressure, a range controller for selectively supplying line pressure to lines corresponding to respective shift ranges, a shift controller for supplying hydraulic pressure from the range controller to lines corresponding to respective shift ranges, a hydraulic pressure controller for duty-controlling hydraulic pressure from the range controller into operational pressure operating the friction elements, and a hydraulic pressure distributor for determining a hydraulic flow path corresponding to each shift range by operating valves according to the hydraulic pressure from the shift controller, and suitably distributing the operational pressure to each friction element.
The shift controller operates spool valves of the hydraulic pressure distributor under the control of a transmission control unit, such that the hydraulic flow path corresponding to each shift range is determined to realize shifting.
When changing shift stages, the timing of exhausting hydraulic pressure from one set of friction elements and supplying hydraulic pressure to another set of friction elements through the hydraulic flow path, determined by the shift controller in accordance with each shift range, greatly influences shift quality. However, mis-timing can cause an abrupt increase in engine revolutions or locking of the shift mechanism. Further, abrupt changes in hydraulic pressure levels in the hydraulic flow path decreases the life span of the automatic transmission.
In order to improve shift quality and durability of the transmission by both accurately controlling the timing of pressure supply and minimizing changes in hydraulic pressure levels, a method of modifying shift valve structure has been developed. However, such a method complicates both the structure of the shift valves and the control process.